It is known to scavenge the exhaust gas tract of gas turbines or gas turbo groups with fresh air after a standstill before the flue gases take action. This avoids the situation where combustible gases, which may potentially accumulate during lengthy standstill phases, ignite when the plant is put into operation. The importance of this scavenging operation comes to the fore particularly when the arrangement of exhaust gas heat exchangers in the flue gas tract of a gas turbo group or of another thermal power engine provides large volumes in which ignitable gas mixtures may potentially accumulate. For example, in combined-cycle power plants, to drive the gas turbo group for several minutes without firing before the gas turbo group is put into operation, in order thereby to generate a flow of fresh air through the waste-heat recovery steam generator. Only when the volume of the waste-heat recovery steam generator has been conveyed, for example, five times by the gas turbo group does an ignition of the gas turbo group take place. On account of the limited maximum throughput through the gas turbo group, such an operation takes up several minutes, typically of the order of ten minutes to 20 minutes. The result of this is that power plants of this type can deliver power to the network, from standstill, only after a comparatively long delay time. In air accumulator plants, too, the stored fluid is in most instances heated in an exhaust gas heat exchanger before it is supplied to the expansion machine. Accordingly, even these plants, which are intended per se for covering peak load and in which short start-up times are therefore desirable, can deliver to the network only after delay.